Myoglobin is an alpha-helical globular protein containing two highly conserved tryptophanyl residues at positions 7 and 14 in the N-terminal region of the molecule. The simultaneous substitution of the two residues with phenylalanine makes the resulting protein (W7FW14F) highly prone to aggregate at physiological pH and room temperature with subsequent amyloid fibril formation. Thus, this mutant is an ideal model to investigate the molecular basis of misfolding and protein aggregation in amyloid diseases caused by point mutations. In this report, we review the molecular mechanism and the kinetics of the aggregation process of W7FW14F apomyoglobin as well as the molecular bases of the intrinsic toxicity of the aggregates. The process consists of hierarchically correlated steps, each one characterized by morphologically distinct intermediates with a different cytotoxic potential. The early aggregation event is the fast assembly of predominantly native-like helical protein monomers into oligomeric species. The next step is a structural re-organization of the monomers inside the oligomers resulting in the conformational change of some regions from helical to β-strand. Finally, the oligomeric species associate into the regular amyloid fibrillar structure. The aggregation intermediates exhibit different cytotoxic potential, the oligomeric species being much more toxic than the mature amyloid fibrils.